The Little Engine That Could

For a long time the smallest motor in the world was 200 nanometers across. That’s really small, about one-fortieth the size of a red blood cell. Charles Sykes and his team at Tufts University have now crushed that rec­ord. Their motor is just a single molecule, one nanometer across. Unlike other, bigger motors, this one isn’t driven by chemical reactions or light—it runs on electricity. “A lot of designs have been proposed,” says Johannes Seldenthuis, a researcher at the Delft University of Technology in the Netherlands, “but this has really been the first one that’s actually worked.”

Here is how Sykes and his team did it: the motor—a single molecule of butyl methyl sulfide (BuSMe), which is made up of a sulfur atom with four carbon atoms on one side and one carbon atom on the other—was placed on a copper surface. The researchers then lowered a scanning electron microscope very close to that surface. The electrons flowed out of the tip of the microscope, exciting the electrons in the BuSMe molecule and causing it to rotate back and forth. But because the molecule is asymmetrical, it tended to spin in one direction slightly more than the other. Little by little, the excited molecule moved along the plate.

Although that might not sound like a motor, it resembles what is inside the human body. “If you watch biological motors, they don’t look like they’re doing anything useful—they just jitter around,” Sykes says. Our bodies are populated with similar structures. The cell wall is littered with pumps that move ions into and out of the cell. Others move things from one place to another within the cell.

That is why this single-molecule motor is significant. Says Sykes: “If you can operate them in a controlled way, you have a whole host of things you can do.” If researchers could reliably build replicas of the pumps in our cell walls, they could do experiments on those pumps extremely efficiently, shrinking their setups to fit onto small chips and reducing the amount of space, cost and time required for each test.